Typically, catheters are made of biocompatible plastics such as polyurethane and thermoplastic elastomer resins, such as Pebax.RTM.. However, when a catheter made from such a material is used during a surgical procedure, the catheter is kept in the body for periods lasting from one to eight hours. During this time, the plastic of the catheter absorbs body fluids and becomes softer over time. Experiments with typical polyurethane and Pebax shafts soaked in body-temperature distilled water confirm this conclusion. For example, it takes almost twice as much force to bend a dry Pebax.RTM. catheter over an angle of 60.degree. compared to a Pebax.RTM. catheter soaking for an hour. Further, it takes more than twice as much force to bend a dry polyurethane catheter over an angle of 60.degree. compared to a polyurethane catheter soaking for an hour. As a result, it becomes difficult for a physician to manipulate the catheter over the course of the procedure because the mechanical properties of the catheter change, with the catheter becoming harder to push, pull and turn. For example, a catheter that absorbs moisture looses its "pushability", i.e., tends to perform like a wet noodle, becoming more difficult to push through the body. Additionally, in the case of catheters having a deflectable distal end, as the catheter shaft absorbs moisture, the deflectable distal end responds differently to the same movement of the handle over time. Further, the ability of the distal end to hold the deflection decreases, and it is more difficult to hold against the tissue. Accordingly, a need exists for a catheter that overcomes these disadvantages.
In an attempt to address some of these disadvantages, U.S. Pat. No. 5,545,151 (the '151 patent) discloses a catheter shaft formed from an inner layer of a particular blend of polyester and polyurethane, an outer layer formed from a different blend of polyester and polyurethane, and a reinforcing means therebetween. The '151 patent states that polyester-polyurethane blends are found not to absorb moisture and, hence, will not change in dimension when immersed in saline, body fluids and/or contrast media. However, a catheter shaft formed from a polyester-polyurethane blend has a particular feel and maneuverability that is different, for example, from a catheter shaft formed from polyurethane or Pebax. Physicians develop strong preferences for catheter shafts having a particular feel and mechanical properties, and are often reluctant to use a catheter having a different feel. Accordingly, a need exists for a way to modify a preexisting catheter shaft to reduce its ability to absorb moisture, while at the same time substantially retaining the mechanical properties and feel of the catheter.